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Full text of "Handbook Of Chemical Engineering - I"

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CONCENTRATION
333
Sand Tables and Concentrators.—Employing principle (2) given at the beginning of this section, the Wilfley table was the modern table of this type and is still one of the leading makes. Figure 5 shows the general arrangement of this table. The surface of the table is covered with linoleum to which riffles are secured. Feed enters at the point A and the separated material fans out, the leanest material leaving the deck at a point somewhere in the neighborhood of point B and the richest in the neighborhood of point C. The i deal arrangement of the discharging material is illustrated diagrammatically by Fig. 6. The black circles are the heavy mineral and the light the gangue. In order to have the clue to the arrangement of the grains, stratification must be explained and denned. This term is a misnomer, for there is no actual arrangement of distinct layers. Actually after the ore and water has reached the deck and come under the advancing shake of the head motion it tends to assume some such arrangement
FIG. 5.-™-Wilfley table.
as is shown by Fig. 7. The riffles on the deck have little effect in causing this arrangement. They aid in holding the water and keeping the mass mobile and of even depth. This arrangement of the grains is caused by interstitial settlement; which has already been explained to some degree in Section VI. The additional effect of specific gravity is to tend to make the heaviest grains pass down and take a position nearer the deck than those of inferior specific gravity. When the ore is spread out sidewise by the inclination of the deck and the influence of the wash water which comes from the water box D, Fig. 5, it tends to assume, where there are two minerals involved in the concentration, the appearance shown in Fig. 6. The part of this figure enclosed in dotted lines should be compared with Fig. 7. The capacity of shaking tables ranges from 5 to 20 tons per 24 hr. One man can attend 20 of them. Owing to the overlap of the bands in making separations on sand tables there cannot be a clean division of the products. Overlapping is greater where a screen feed is placed on the table and less where a water classified feed is employed since this tends to produce small grains of heavy mineral and large grains of coarse ones. Expressed diagramatically it would tend to bring heavy grains such as E, F,